1. Field of the Invention
The present invention concerns a method for the metallization of a luminescent screen. More particularly, it concerns a method for depositing an organic film, which can be eliminated by heat, on the luminophors of a luminous screen used especially in color television tubes or color display monitors.
2. Description of the Prior Art
As described in the U.S. Pat. No. 3,582,390, one of the commonly used methods for the metallization of a luminophor screen for cathode ray tubes consists in producing, on the surface of the luminophor screen, a volatilizable substrate which can be obtained from an aqueous emulsion of a water-insoluble film-forming resin, depositing a metallic layer on the substrate and then eliminating the substrate by volatilization. To enable the substrate to be volatilized, the metallic layer should therefore have sufficient porosity. Hence, to adjust the porosity of the metallic layer, the aqueous emulsion contains a small quantity of hydrogen peroxide and an organic polymer forming a water-soluble film. Other additives, such as colloidal silica, can be added to the emulsion. The additives improve adhesion and reduce the formation of blisters in the metallic layer.
This manufacturing method has proved to be quite satisfactory for making the dot type screens which used to be manufactured until the nineteen seventies. Since then, a striped type of screen structure has come into use in industry. With screens of this type, the irradiation systems used are highly sophisticated and, hence, very expensive. Hence, to reduce the initial investment, far more sensitive slurries are used to obtain the luminophor. In this case, the basic photo-polymer system still has polyvinyl alcohol as a binder along with an ammonium, sodium or potassium dichromate as a photo-sensitizer. However, to increase the sensitivity of the luminophor slurry to ultraviolet light by a factor of about 2, substantial quantities of additives, such as ethylene glycol, triethylene glycol and methyl pyrrolidone are added to this slurry. Unfortunately, these additives also act as softeners of the photo-polymer and leave a polymer film with a soft consistency in the luminophor stripes. Thus, after the luminophor screen is deposited, the polymer in the luminophor stripes has a soft consistency which depends on the color depositing sequence. Thus, when the volatilizable substrate is made, when additives such as hydrogen peroxide are used in the aqueous resin emulsion forming a film, as described in the U.S. Pat. No. 3,582,390, the etching of the polymer by hydrogen peroxide becomes highly selective depending on the sequence in which the luminophor colors are deposited. This results in an unequal tightness of film and a loss of luminescence of the tube occurs, at least for the last color deposited, due to over-etching by hydrogen peroxide.
An aim of the present invention is to overcome these drawbacks and to prevent the selective etching of the polymer in the luminophor stripes of different colors.